1. Field of the Invention
The present invention relates to glass sheet bending/tempering procedures and more particularly to the covering of annular molds used in such procedures.
2. Description of the Related Art
Horizontal bending/tempering procedures generally involve a first bending phase, where the glass sheet previously heated in a furnace to beyond its softening point is, e.g., applied by pneumatic means at least once to an upper member constituted by a refractory ceramic or sheet metal plate. In certain cases, this member is only a carrying means. In other cases it constitutes a shaping or preshaping means for the glass sheet and has a principal curvature which can be more or less close to that which it is wished to give to the glass sheet. However, no matter what its shape and function, hereinafter it will be designated by the term plate bending mold.
Thus, the sheet can be sucked, as is described in EP-A-3,391 and/or raised by an upward, hot gaseous flow, against the bending mold in the manner described in FR-A-2,567,508. It can also be raised by mechanical means such as lugs or a lower mold able to have different configurations, as is described in FR-A-2,009,278.
The more or less preshaped glass sheet can then be deposited in a well known manner on a mold having a curved profile open in its center and which is known as an annular bending bold, before being discharged while on the latter towards the thermal tempering zone. This tempering operation consists of supplying, from two plenums positioned above and below the glass sheet on the annular mold, also called a tempering frame, cooling gas jets which are approximately perpendicular to the two faces of said sheet.
The tempering frame is provided with a coating or covering specifically adapted to this type of tempering and to the stress which is imposes. Thus, the covering, apart from being able to withstand temperatures between 650.degree. and 800.degree. C. and without marking the glass, must be permeable to air so as not to disturb the circulation of the pressurized cooling air from either side of the glass sheet. Thus, the air projected from the lower tempering plenum with respect to the annular plenum supporting the sheet must be able to also flow tangentially along the contact surface between the mold and the sheet, so that the periphery of the sheet is also correctly tempered, which requires a relatively permeable mold covering in order to permit said flow. Various such coverings provided for the tempering frames are known in the art.
It is standard practice to use a refractory fiber or thread gauze with relatively wide meshes of approximately at least 1 mm side length and optionally in the manner disclosed in U.S. Pat. No. 4,906,271, while fixing the meshes by knots at the intersection points, so as to render uniform the distribution of the intersection points which constitute the essential contact points between the mold covering and the glass sheet.
It is also known to complete the shaping of the glass sheet by pressing the sheet between the plate mold above it and an annular bending mold below it. To the extent that pressing involves considerable and relatively sudden mechanical forces imposed on the glass sheet at its periphery supported by the annular mold, the annular mold covering must have very specific characteristics different from those required for the covering of a tempering mold.
Thus, it must have a certain softness so as to permit a gentle contact with the glass sheet and cancel out any slight imperfections of the shape of the mold. It must also be relatively deformable and extensible, so that it can be easily fixed while stretching it over the mold, e.g., using clips or hooks. It must be able to absorb the dust which might be located at the mold/sheet interface and have a certain porosity, so that the dust cannot mark the surface of the softened glass. It must also not mark the glass by its own surface texture.
The covering must therefore have a relatively dense and compact texture, such as is encountered with metallic knitted products with compact meshes, as disclosed in FR-A-2,644,156 and which makes it possible to obtain a contact surface with the glass sheet which is as smooth and level as possible. Thus, during the relative sliding between the glass sheet and the annular mold, there is no risk of marking or scratching the glass sheet. During the actual pressing between the plate mold and the annular mold, such risks are obviated by ensuring a gentle, shock-free contact between the glass and the annular mold.
However, this type of dense texture covering is incompatible with a use as a covering for a tempering mold because, as has been stated hereinbefore, it would not permit an effective tempering of the periphery of the glass sheet, by opposing the tangential flow of the cooling air. It has therefore been necessary to use for tempering and for pressing two annular molds having coverings of different types. However, this involves a supplementary mold exchange stage with all the problems of synchronization and increases in the cycle time which result therefrom.
It is possible to envisage carrying out the glass sheet pressing operation with the frame designed and covered for the thermal tempering operation, but this remains a compromise which is not acceptable from the standpoint of the optical quality of the glazings obtained except when the relative sliding between the glass and the annular mold which leads to the production of marks is very limited, i.e., when only a modest bending has to be given to the glass sheet.
A "bifunctional" annular mold which is suitable for relatively pronounced bending operations without marking the sheets is described in EP-A-373,992. It is an annular mold having a part fixed with respect to the mold support frame and a rail articulated with respect to said fixed part which, on bending back during the pressing against the upper plate mold, makes it possible to obtain glass sheets which are locally highly bent. The fixed part is covered with a gauze having relatively wide meshes allowing tempering to take place, whereas the articulated rail is covered with a dense felt suitable for pressing. During tempering, the same annular mold is used, but then the rail is in the opened out position, so as not to disturb the tempering operation at the highly bent edge of the sheet. This consequently means that the marginal area is left without any support during tempering. This situation is liable to produce parasitic deformations following pressing and account must be taken thereof, e.g., with respect to the curvature of the annular mold and/or the extent of the pressing by the articulated rail, so as to obtain an end product with the desired curvature.
By its very design, this mold is only usable if the desired highly bent area is not very extensive and is limited to a single side of the glass sheet. It is necessary that the glass sheet be supported during tempering by a relatively large fixed part so as to prevent any sagging thereof, said fixed part also being slightly curved and covered with a tempering fabric, pressing essentially taking place via the articulated rail. Consequently this mold is unsuitable for glazings having a complex shape, e.g., those having curvatures in different directions close to two opposite or adjacent edges.